WORST_CASE(?,O(n^1))

Solution:
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  "0" :: [] -(0)-> "A"(2)
  "f" :: ["A"(2) x "A"(0)] -(1)-> "A"(2)
  "s" :: ["A"(2)] -(2)-> "A"(2)


Cost Free Signatures:
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Base Constructors:
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